The need to have hard masks for metal etching has become critical for the semiconductor industry. The need is driven by the depth of field requirement of the lithography process. Since higher numerical aperture (NA) is used for sub-micrometer processing, the depth of field of the resolution drops proportionally. Therefore, to obtain good process control, a thinner resist must be used. However, thinner resists cause metal etching problems, because the metal etch chemistry exhibits poor selectivity to the resist. This, in turn, causes difficulties in control of critical dimensions (CD).
A hard mask has been devised by some workers in this field to combat this problem. The hard mask could be silicon dioxide or silicon nitride. Typically, the hard mask is deposited by plasma enhanced chemical vapor deposition (PECVD). The accepted industry process operates at about 350.degree. to 400.degree. C., which is adequate if the hard mask is a "stand-alone" mask (i.e., the mask, not being a photoresist material, does not have any interaction with previous processing steps).
An aluminum-based alloy is still the material of choice for the interconnects which makes electrical contact to metal plug contacts in silicon-based semiconductor devices. These Al alloys may be doped with Si, Cu, or Ti, or may be pure Al. However, if these Al alloy films as deposited are subjected to a high temperature treatment, then they exhibit a phenomenon called "sunken grain". The reason for this behavior is related to thermal stress. When the film is in sheet form, there is no X-Y movement possible; the only direction for stress-relief is in the Z-direction. When the Al alloy film is heated to high temperature, higher than its original deposition temperature, then the film will be under zero stress. However, during cooling, tensile stresses develop. Since the film is restricted in the X- and Y-directions, then it can only relax in the Z-direction. This relaxation forms "voids" inside the aluminum film, which are associated with the "sunken grain" phenomenon. These void show up as "rings" on the substrate after the aluminum is etched.
After the aluminum film is patterned and etched to form the metal interconnects, then a passivation layer is deposited on the metal interconnects. The metal interconnects are then compressed in all three directions: X, Y, and Z. In this case, voids will not form unless the device is stored at a temperature in the range of about 150.degree. to 200.degree. C. for an extended period of, say, 1,000 hours.
Voids will form inside the aluminum-containing layer under high temperature conditions, whether the aluminum is doped or undoped, regardless of whether the aluminum sheet is covered by a conventional TiN anti-reflection coating (ARC) or not. If no TiN ARC is in place, these defects, which will be transferred onto the oxide substrate as "rings" upon completion of the metal etch, may not be detrimental. However, if the metal stack to be etched has TiN or another anti-reflection coating formed on the Al-containing layer, then, due to the interaction of ARC and Al around the void region, the ring defects on oxide surfaces become difficult to etch and can cause metal bridging.
Thus, a process for using a hard mask for patterning the aluminum-containing interconnects that avoids the foregoing problems is required.